I know that the OEM boys. Ford, GM, Nissan, etc. always deal with hormonics in a chassis. I had a collision shop for over 30 years. A Nissan Pathfinder has a 3 lb weight cantilevered off it's right rail about mid way under the "B" center piller post. It's there to counteract the hormonics engerated throughout the chasiss. The shaking/shimmy is a common problem among all brand of Cobra replicas, and I believe is related to hormonics, and may be lessened by doing some of the things spelled out.
I just dug this out of my files and thought if might shed some light on this topic.
The 65 MPH "Shimmy"
This is fairly common although it is usually corrected fairly easily.
First and foremost, be sure your tires are properly balanced. It must be done by a competent technician using a dynamic balancer. Some shops perform dynamic balancing, but they don't do it correctly. It should be done using a more sensitive setting and the technician should properly install the balancing weights. It should also be re-checked after weight installation. In addition, most major tire manufacturers put a marking on each new tire to indicate the "heavy spot" on the tire. Kevin Lakkis, field development engineer from Toyo Tire, notes:
Toyo measure uniformity – Radial Force Variation, Lateral Force Variation, Balance, etc. AND no tire leaves the factory beyond certain limits. (That is why Toyo has so few vibration complaints).
Every Toyo tire is marked at the low balance point with a yellow dot (it is not always a yellow dot with all tire manufacturers, but it is with Toyo). OE tires are also marked with a red dot, marking the low RFV spot (so they can be match-mounted with the rim).
A general rule is to match the yellow dot to the valve stem, however, many wheel manufacturers drill the valve stem hole at the light spot of the wheel.
In any case, when I mount my own tire, I spin up the wheel alone, mark the heavy spot, and match that to the yellow dot on the tire. In my limited experience, the heavy spot of the wheel is not always near the valve stem.
BTW, there is a new balance machine that puts a load roller wheel on the tire and actually measures RFV as well as balance. We are ordering one very soon. That type of machine should help with the 65mph shimmy. I plan to work with my local chapters with people who can’t seem to get rid of the vibration, and note exactly how sensitive the Miata is.
Check out this link:
www.GSP9700.com
We (Toyo) have been using one of these machines for some time with excellent results.
We use it to diagnose vibration complaints.
There is a dealer locator which will help Miata owners find a tire dealer who has one of these special balancing machines. Since the GSP9700 is significantly more expensive than standard balance machines, it is safe to assume a tire dealer who owns one is an expert on diagnosing vibration problems.
Finally, if all else fails, take the tires to a specialist and have them "shaved" to 100% true. Manufacturing anomalies are not as uncommon as one would think. Shaving the tires will bring them back into the "round."
Next, the car should be properly aligned by an alignment specialist. Just because the car is new, don't assume that it has been properly aligned at the factory or by the dealer. Many dealers are not very good at alignment. Go to a specialist! If you want to get a very accurate alignment, tell them to let you sit in the driver's seat while they're doing it. The additional weight on one side of the car affects the alignment dramatically.
Consider installing aftermarket bracing to help reduce frame flexing. A rear subframe brace was standard from '92 to '96(?), but can be added to earlier models. A front subframe brace can be added as well as a shock tower brace. The combination of steps should eliminate the shakes.
If that doesn't take care of it, check out Dunlop Tire's description on troubleshooting vibrations.
Here's another tip that was sent in by Brent Tomerlin
I work with a large tire company and over the years I have had plenty of people with "uncurable" balance problems. These were on all cars, not just Miatas. They best solution I have found is to have the tires match mounted. This can be done with a balance machine with a match mount program. What happens is through a process of certain steps the machine figures out the high and low spots of both the wheel and the tire. It then instructs you where to line them both up. This will decrease the weight needed and also give the smoothest ride possible. I've also found that once you air the tire up the final time it's best to deflate once more just in case any air is trapped between the tire and wheel. This is a tedious process as you must semount the tire 3 times for each wheel and usually takes about an hour or so for 4 tires. Maybe two or three times in 6 1/2 years have I had somebody still have a vibration problem.
posted by Doug Reed, Reed Peformance, SPF dealer
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